A growing area in the field of printing is known as flexography or flexographic ink printing. Flexographic ink printing is a branch of rotary typographic printing in which the printing is applied to a print-receiving substrate by use of a flexible relief plate with highly fluid, volatile inks which dry rapidly by evaporation. The process was originally limited to printing paper bags, but many inroads have been made in recent years to extend the process to printing various flexible packaging, textiles, newsprint, and other print-receiving substrates.
A flexographic printing ink generally contains three basic types of ingredients denominated as a solvent, a colorant, and a binder. Minor amounts of other additives also may be employed to provide the desired characteristics (i.e., rheology, viscosity, etc.) for the printing ink composition.
In recent years, increased emphasis has been placed on the use of water as the solvent, rather than an oil-based solvent. Carbon black pigment is commonly used as the colorant in both water-based and oil-based black ink formulations.
The binder component of the printing ink composition generally comprises a resin which functions primarily to increase adhesion of the pigment to the substrate, while also often acting as a dispersing medium and carrier. Acrylic polymer lattices have traditionally been employed as binders in water-based printing ink compositions. However, these acrylic binders are relatively expensive. Thus, printing ink formulators have actively sought low-cost alternative binders.
One promising source of alternative binders under evaluation is lignin. Lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. In the paper-making industry, lignin may be recovered as a by-product of the cellulose product. Depending on conditions under which the lignin is precipitated, the precipitated lignin may be either in the form of free acid lignin or a lignin salt. If lignin is precipitated at a high pH, such as about 9.5 to 10, the lignin is obtained in the form of a salt. If this lignin is further processed by washing, acidifying to a low pH, such as about 2 to 5, and further washed so as to be substantially free of salt and ash-forming ingredients, free acid lignin, known as "A" lignin, is obtained. A monovalent salt of lignin, such as an alkali metal salt or an ammonium salt, is soluble in water, whereas free acid lignin and polyvalent metal salts of lignin are insoluble in water.
It is known to employ certain amine salts of lignins as binders in aqueous printing ink compositions, as shown in commonly assigned U.S. Pat. No. 4,891,070.
It is also known to use certain lignins as binding agents for water-based ink systems incorporating pigments and/or dispersed dyes. U.S. Pat. No. 2,525,433 discloses the use of lignin dissolved in a water-miscible solvent as a binding material for a pigment in printing ink. The method of U.S. Pat. No. 2,525,433 requires the use of a solubilizing agent, generally an organic amine or alcohol, to completely dissolve the lignin particles. The resultant solution when used with pigments for ink application is limited to newsprint or other porous paper. There is no film-forming action of the lignin, and no solid particles remain after the lignin is solubilized. Adhesion to coated paper, calendared paper, and sized papers is poor with excessive rub-off resulting.
Commonly assigned U.S. Pat. No. 5,008,378 (which is hereby incorporated by reference) teaches a method of producing lignin materials which, in liquid dispersion form, comprise particles having a median particle diameter of less than about one micron, and of such uniform shape and size so as to be readily employed as film-formers and additives in other products and chemical compositions. The lignin material product comprises colloidal dispersion of monodispersed lignin particles which coalesce and dry to form a continuous film which is useful as a binder in pigment printing, coating, and composite board manufacture.
Commonly assigned U.S. Pat. No. 4,957,557 (which is hereby incorporated by reference) teaches the use of this submicron or small particle lignin to produce carbon black ink compositions. However, a major problem has recently been discovered in that inks prepared with this type of lignin are not storage stable (particularly at elevated temperatures). Over time viscosity increases and sediments are formed or gel-like inks are obtained which are extremely difficult to dilute to desired press-ready viscosities (commonly about 10 seconds as measured via a #3 Shell Cup or about 20 seconds via a #2 Zahn cup).
Therefore, it is the object of this invention to provide an improved low cost small particle size lignin binder for use in producing storage stable, low viscosity flexographic water-borne black ink formulations.